1. Field of the Invention
The present invention relates to an information processing apparatus having a dot density conversion function, and more particularly to a dot density conversion method and system suitable when an conversion object consists of at least one image data such as a text, a graphic, an image, or the like.
2. Description of the Related Art
When image data is outputted to an output device in an information processing apparatus handling image data, the image data is converted to a dot density of the output device.
An example of a conventional information processing apparatus having a dot density conversion function is disclosed in Japanese Patent Laid-Open No. 54867/1988. This apparatus stores the inputted image data as the image data of one page and applies dot density conversion to this image data of one page uniformly.
Conventional dot density conversion methods include an SPC (Selective Processing Conversion) method which calculates the distance between a conversion dot and each original dot and uses the value of the closest original dot as the value of the conversion dot when the conversion dot is positioned inside four adjacent original dots, a logical OR method which calculates the value of the conversion dot by the logical OR of the value of each original dot, and the like. Furthermore, there are a method which skips merely the dots at each predetermined intervals, a method which calculates the logical sum with an adjacent dot when the dot is skipped and reflects the value of the dot to be skipped on the adjacent dot, and the like.
However, the prior art technique described above involves the following problems.
The inputted image data consists of dot groups representing the whole page and the dot density conversion processing is effected uniformly for the dots of the whole page. Therefore, degradation of image quality such as the occurrence of moire and a density change occurs at bi-level photographic image portions and irregularity of lines such as the increase and decrease of line widths occurs at graphic portions. Although either the SPC method or the logical "OR" method is capable of obtaining a relatively good conversion result, since the conversion processing is effected for the dots of the whole page, the processing time is increased and a hardware configuration must essentially be employed for the conversion processing to limit the processing time to a practical level. If a simple conversion algorithm such as mere skip of dots is employed to improve the processing time by the software alone, degradation of image quality is unavoidable.
As described above, the conventional methods cannot easily obtain a good conversion result at each portion of a text, a bi-level photographic image, a graphic, etc, that exist in mixture in the image data, and cannot either accomplish practical image quality and a conversion speed by softwares.